Key points We used the idea of synergic control and the framework of the uncontrolled manifold hypothesis to explore the synergic control of a single muscle. Individual motor units in flexor digitorum superficialis formed two‐three groups (MU‐modes) with parallel changes in firing frequency, robust over force‐up and force‐down segments. There were strong force‐stabilizing synergies in the MU‐mode space during accurate cyclical force production. The results show, for the first time, that the idea of synergic control is applicable to individual muscles. The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action. Abstract In the present study, for the first time, we have used the idea of synergic control and the framework of the uncontrolled manifold (UCM) hypothesis to test two hypotheses: (i) individual motor units are organized into stable groups (MU‐modes) with parallel scaling of firing rates with changes in the muscle force, and (ii) the gains of MU‐mode involvement co‐vary to stabilize the desired force magnitude. Young healthy subjects performed accurate cyclical force tracing tasks at 1 Hz by pressing with intermediate phalanges; only one finger was instructed to produce force at a time. Surface electromyographic signal from flexor digitorum superficialis was recorded and used to identify individual motor units and their firing frequencies. Principal component analysis with rotation and factor extraction was used to identify MU‐modes, which showed similar compositions over the force‐up and force‐down task segments. Inter‐cycle variance analysis in the MU‐mode space confirmed the existence of strong synergies stabilizing finger force. There were no synergies stabilizing MU‐mode magnitude in the space of individual motor units. This is the first application of the UCM framework to the neural control of a single muscle. It extends the applicability of this approach to analysis of individual muscles. We discuss the importance of the findings for the idea of hierarchical control and the notion of muscle compartments. The results suggest that segmental spinal mechanisms, such as recurrent inhibition and stretch reflex, probably play a major role in the synergic control of action.

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When stopping a closing door or catching an object, humans process the motion of inertial objects and apply reactive limb force over short period to interact with them. One way in which the visual system processes motion is through extraretinal signals associated with smooth pursuit eye movements (SPEM). We conducted three experiments to investigate how SPEM contributes to anticipatory and reactive hand force modulation when interacting with a virtual object moving in the horizontal plane. Participants held a robotic manipulandum and attempted to stop an approaching simulated object by applying a force impulse (area under force-time curve) that matched the objects virtual momentum upon contact. We manipulated the objects momentum by varying either its virtual mass or its speed under free gaze or fixed gaze conditions. We examined gaze variables, timing of hand motor responses, anticipatory force control, and overall task performance. Our results show that extraretinal signals associated with SPEM are important for anticipatory modulation of hand force prior to contact. However, SPEM did not seem to affect the timing of the motor response or the task performance. Together, these results suggest that SPEM are important for anticipatory control of hand force prior to contact and may also play a critical role in anticipatory stabilization of limb posture when humans interact with moving objects.

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